The present invention relates to an ink jet recording apparatus of the type which ejects ink from a nozzle while applying oscillation thereto, selectively charges ink drops by a charging electrode located in a position where the ink from the nozzle separates into a drop, deflects charged ink drops by a deflection electrode to cause them to impinge on predetermined positions on a sheet. More particularly, the present invention is concerned with a control over the pressure of ink in a recorder of the type mentioned.
In an ink jet recorder of the type concerned, the sheet is usually spaced a relatively long distance from the ink ejection nozzle. Therefore, the ink pressure is designed high enough for ink drops to stably fly as far as the sheet despite the influence of the charging and deflecting electric fields. Stable and accurate controls over the ink viscosity and pressure, oscillation pressure, charge amount, deflection electric field and the like is another important consideration for regularly forming drops of a predetermined size and causing them to accurately fly predetermined deflection paths. Further adequate charging of drops is unattainable unless a charge voltage or pulse is applied precisely timed to the separation of a drop from the ink.
In light of the above, there has been employed a phase search before a record charge control in order to stabilize the ink to a predetermined pressure and/or a predetermined viscosity while predetermining a timing for the application of charge pulses. For the phase search, a contact or non-contact type charge detector electrode is connected to a charge detector circuit whose major components are an amplifier, an integrator and a comparator. A charge pulse having a short duration is applied to the charging electrode, and the phase of the charge pulse relative to the separation of an ink drop is sequentially shifted. When the charge detector circuit produces an output declaring "charged", the instantaneous phase of the charge signal is predetermined to be the adequate charging phase.
The deflection is effected by a flying velocity of an ink drop (hereinafter referred to as drop flight velocity). An expedient heretofore proposed for overcoming this problem is to detect a drop flight velocity and to adjust an ink pressure until the actual velocity coincides with a predetermined target velocity.
In the proposed ink velocity detection and ink pressure adjustment system, a desired result will be achieved if the ink pressure is stabilized at a predetermined level. In practice, however, because the ink is compressed by a pump, the fluctuation of ink pressure corresponding to the periodic pulsation of the pump delivery pressure shows itself, if a little, in the ink inside the ink ejection head even though passed through an accumulator. While the ink pressure fluctuation may be completely eliminated by enlarging the capacity of the accumulator, a larger accumulator capacity is accompanied by a longer period of time while the ink pressure downstream of the accumulator takes to become stabilized at a predetermined level after a change in the delivery pressure of the pump. This translates into a disproportionate period of time for ink pressure adjustment in which the ink pressure is varied while detecting a drop flight velocity, so that the velocity finally converges to a target velocity. Therefore, a certain degree of fluctuation is inevitable in the pressure of ejected ink resulting in irregularity in the detection of ink velocity, error in the ink pressure adjustment, failure in setting a desired ink velocity, etc.
Another drawback inherent in the prior art apparatus is that not only drop flight velocities lower than a certain limit cannot be detected, but a velocity detector circuit is constructed to integrate the flight time of a plurality of charged ink drops and, based on the result of integration, determine whether or not the velocity is adequate, consuming a substantial period of time.
In another known velocity detection system, two static induction type electrodes are positioned along an ink flight path. The drop flight velocity is determined by counting a time which a charged ink drop takes to be detected by the second electrode after being detected by the first. A problem encountered with this type of system is the poor reliability of operation due to a low detection accuracy particular to a static induction type electrode and progressive deterioration of accuracy. For a higher accuracy, the distance between the nozzle and the gutter, i.e., the distance the ink drops fly, has to be increased. Accuracy is also required in the physical installation of the detection electrode which renders the construction intricate.